netdev - Re: [PATCH v3 bpf-next 1/3] bpf: switch BPF UAPI #define constants used from BPF program side to enums

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Message-ID: <945cf1c4-78bb-8d3c-10e3-273d100ce41c@iogearbox.net>
Date:   Wed, 4 Mar 2020 16:38:01 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH v3 bpf-next 1/3] bpf: switch BPF UAPI #define constants
 used from BPF program side to enums

On 3/4/20 10:37 AM, Toke Høiland-Jørgensen wrote:
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>> On Tue, Mar 3, 2020 at 3:01 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>>
>>> On 3/3/20 1:32 AM, Andrii Nakryiko wrote:
>>>> Switch BPF UAPI constants, previously defined as #define macro, to anonymous
>>>> enum values. This preserves constants values and behavior in expressions, but
>>>> has added advantaged of being captured as part of DWARF and, subsequently, BTF
>>>> type info. Which, in turn, greatly improves usefulness of generated vmlinux.h
>>>> for BPF applications, as it will not require BPF users to copy/paste various
>>>> flags and constants, which are frequently used with BPF helpers. Only those
>>>> constants that are used/useful from BPF program side are converted.
>>>>
>>>> Signed-off-by: Andrii Nakryiko <andriin@...com>
>>>
>>> Just thinking out loud, is there some way this could be resolved generically
>>> either from compiler side or via additional tooling where this ends up as BTF
>>> data and thus inside vmlinux.h as anon enum eventually? bpf.h is one single
>>> header and worst case libbpf could also ship a copy of it (?), but what about
>>> all the other things one would need to redefine e.g. for tracing? Small example
>>> that comes to mind are all these TASK_* defines in sched.h etc, and there's
>>> probably dozens of other similar stuff needed too depending on the particular
>>> case; would be nice to have some generic catch-all, hmm.
>>
>> Enum convertion seems to be the simplest and cleanest way,
>> unfortunately (as far as I know). DWARF has some extensions capturing
>> #defines, but values are strings (and need to be parsed, which is pain
>> already for "1 << 1ULL"), and it's some obscure extension, not a
>> standard thing. I agree would be nice not to have and change all UAPI
>> headers for this, but I'm not aware of the solution like that.
> 
> Since this is a UAPI header, are we sure that no userspace programs are
> using these defines in #ifdefs or something like that?

Hm, yes, anyone doing #ifdefs on them would get build issues. Simple example:

enum {
         FOO = 42,
//#define FOO   FOO
};

#ifndef FOO
# warning "bar"
#endif

int main(int argc, char **argv)
{
         return FOO;
}

$ gcc -Wall -O2 foo.c
foo.c:7:3: warning: #warning "bar" [-Wcpp]
     7 | # warning "bar"
       |   ^~~~~~~

Commenting #define FOO FOO back in fixes it as we discussed in v2:

$ gcc -Wall -O2 foo.c
$

There's also a flag_enum attribute, but with the experiments I tried yesterday
night I couldn't get a warning to trigger for anonymous enums at least, so that
part should be ok.

I was about to push the series out, but agree that there may be a risk for #ifndefs
in the BPF C code. If we want to be on safe side, #define FOO FOO would be needed.

Thanks,
Daniel